A Baltimore judge has refused to dismiss charges against six police officers accused over the death in April of a black man who was in their custody in the US state of Maryland.

During a pre-trial hearing on Wednesday, Circuit Court Judge Barry Williams denied a defence motion for the charges to be dropped against the officers in the death of 25-year-old Freddie Gray, who endured a severe spinal cord injury while in police custody on April 12 and died a week later.

Defence lawyers had sought to drop the charges, which include second-degree murder, because of prosecutorial misconduct on the part of State's Attorney Marilyn Mosby.

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Williams said that while he was troubled by Mosby's public comments about the case, they were not likely to prejudice a jury.

Andrew Graham, a lawyer representing Officer Caesar Goodson, had unsuccessfully argued that Mosby's comments after filing charges against the officers were "reckless and unprofessional", and violated the rules of conduct.

Graham likened Mosby's comments on the case to a "pep rally calling for payback".

Williams was later to rule on a motion for Mosby to recuse herself due to what defence lawyers characterise as conflicts of interest.

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Officers Edward Nero, Garrett Miller, William Porter and Goodson, as well as Lieutenant Brian Rice and Sergeant Alicia White, face charges in Gray's death. They did not attend the hearing.

All the officers face second-degree assault, reckless endangerment and misconduct in office charges.